The present invention relates to a hydraulic gear pump of the type which includes a toothed internal gear and a mating toothed external or pinion gear, and wherein the teeth define fluid cells which are alternately expanded and compressed upon rotation of the gears. In such pumps, the meshing teeth typically have a contact ratio equal to or greater than 2, with the contact ratio being defined as the average number of pairs of teeth in contact.
Hydraulic gear pumps of the described type are commonly employed as regulating pumps for hydraulic fluids. In such cases, the pump includes a plurality of fluid outlet openings which communicate with the fluid cells as they are compressed. The circular pitch of the outlet openings is smaller than or equal to the tooth pitch of the gears. Also, the outlet openings lead to a common discharge duct, or to groups of discharge ducts, and each outlet opening typically includes a non-return valve, although the outlet opening closest to the pitch point may not have a non-return valve.
The above described gear pumps are designed and constructed to have a delivery output characteristic which is speed dependent up to a limiting value, and wherein beyond the limiting value, the output remains substantially constant irrespective of the speed. The limiting speed can be adjusted by positioning an adjustable throttle in the fluid inlet line leading to the pump.
German OS No. 34 44 859 discloses a speed dependent gear pump wherein a contact ratio of at least 2 exists, so that the teeth of the pump form at least 2 and preferably 3 or more fluid cells which are sealed from each other on both the suction and the pressure sides.
In comparison with other known regulating pumps, wherein the delivery characteristic is not speed dependent, or in pumps where the delivery output is adjustable independently of the speed, the pump of the present invention has an advantage of being of sturdy construction, and which permits adjustment of the delivery output characteristic without expenditure for additional mechanical means. The pump of the present invention is particularly desirable for use as the regulating oil pump in an automobile engine, wherein the speed fluctuates constantly. In such uses, the pump serves as a hydraulic pump or the lubricating oil pump, since in these pumps the maximum discharge may be limited without loss in efficiency at a predetermined relatively low speed.
It is also an object of the present invention to provide an internal gear pump of the described type, and which is operable by a reduced power requirement as compared to conventional pumps of this type.